The polyphagous shot hole borer (PSHB), an ambrosia beetle known as Euwallacea sp., has become a serious pest in southern California, threatening urban, forest, and agricultural hardwood trees and shrubs. Notoriously difficult to spot and equally difficult to kill, these beetles are very small, about the size of a sesame seed, and have been observed on over 200 host species. The females bore into healthy hosts and disperse a fungus (such as Fusarium euwallacea) they carry with them as they make their galleries. The fungus is the insect’s food, but it is pathogenic to the plant as it spreads through the plant's xylem. The infection blocks the flow of water and nutrients, causing a range of symptoms from staining and gumming to Fusarium dieback and fatality.